Field of the Invention
The present invention relates to a fixing apparatus including a fixation cooling unit, such as a cooling fan, in order to suppress a temperature rise in a non-sheet-passing area for a small-size recording medium of a fixing member for heat-fixing a toner image, and an image forming apparatus having the same.
Description of the Related Art
In general, a fixing apparatus for fixing a toner image formed on a recording medium thereto is widely employed for various kinds of apparatuses including an image forming apparatus, such as a copying machine, a laser beam printer, or a facsimile machine. The fixing apparatus, in which a fixation area is defined at a nip portion between a heating member functioning as a fixing member and a pressure member abutted against the heating member, applies a heating action of a heat source provided in the heating member and applies a pressurizing action of the pressure member while conveying the recording medium on which the toner image is formed into the fixation area, thereby fixing the toner image onto the recording medium. At this time, in order to achieve satisfactory fixation of the toner image, that is, a satisfactory fixing property, in the above-mentioned fixation area, fixation parameters including a heating temperature (controlled temperature) in the fixation area and a recording-medium conveying speed (process speed) need to be satisfactorily adjusted. Therefore, an energization state of the heat source (heater) provided in the heating member functioning as the above-mentioned fixing member is controlled by a fixing-temperature controller provided in an apparatus main body to thereby perform fixing-temperature control.
Here, in a case where the recording medium of a small size is conveyed into the fixation area, the recording medium does not pass, for example, both end parts in a longitudinal direction, and hence the both end parts become a non-sheet-passing area for the small-size recording medium. In a case where such recording media of the small size are used continuously, temperature transitions in the respective portions on the fixing member are obtained as illustrated in FIG. 10. More specifically, the temperature of a sheet-passing area (central portion) indicated by the solid line of FIG. 10 stays around a set target temperature Tα for the fixing-temperature control with stability by the heat-absorbing action of the recording medium being passed, while a non-sheet-passing portion that is not passed by the recording medium comes to a heat-storage state due to an absence of heat absorbed by the recording medium. As a result, the temperature of the non-sheet-passing area for the small-size (end portion) recording medium indicated by the broken line of FIG. 10 continuously rises until a time t4 at which continuous passing of the recording media is finished, which may cause a phenomenon called a non-sheet-passing-portion temperature rise or an end-portion temperature rise.
The non-sheet-passing portion temperature rise or the end-portion temperature rise thus caused in the case where the recording medium of the small size continues may lead to damage to the heating member functioning as the fixing member and the pressure member, and hence conventionally, for example, the heating member functioning as the fixing member is provided with a fixation cooling unit including a cooling fan, the fixation cooling unit being located in both end parts in the longitudinal direction serving as the non-sheet-passing areas for the small-size recording medium. With this structure, the cooling-down operation is performed when the recording medium of the small size is used, thereby attempting to suppress the above-mentioned non-sheet-passing-portion temperature rise or the end-portion temperature rise. Such a cooling-down operation state of the fixation cooling unit (cooling fan) and the energization state of the heat source provided in the heating member functioning as the fixing member are controlled by the fixing-temperature controller provided in the apparatus main body, and so-called fixing-temperature control is performed. In particular, Japanese Patent Application Laid-Open No. H04-51179 and Japanese Patent Application Laid-Open No. 2003-076209 each disclose a fixation cooling unit (cooling fan) which can adjust the length in a width direction of a fan opening in accordance with the width dimension of the recording medium being used, thereby preventing the non-sheet-passing-portion temperature rise from occurring in the recording medium having a different size.
On the other hand, when a recording medium of a large size is passed immediately after the recording media of the small size are continuously passed, sheet passing is performed in a non-sheet-passing-portion area exhibiting a high temperature, and hence a so-called high-temperature offset phenomenon is likely to occur. Therefore, Japanese Patent Application Laid-Open No. 2003-173103 discloses an apparatus which discontinues an image-forming operation and enters a post-rotating operation at the time t4 at which the passing of the recording media of the small size are completed and maintains the cooling-down operation state of the fixation cooling unit (cooling fan) with the sheet of the large size to be fed being in a standby state and with energization of the heat source within a heat-fixing member being in an interrupted state. By such a standby-control operation during post-rotating, the temperature of the non-sheet-passing area for the small size is caused to decrease rapidly as indicated by the curved line X and to agree with a temperature T0 in the sheet-passing area between the above-mentioned time t4 and a time t5. Then, the energization of the heat source within the fixing member is started at the time t5 at which a substantially uniform-temperature distribution state in which both the temperatures thus agree with each other is achieved, and the temperature of the entire fixing member is raised in a substantially uniform state until a time t6 to reach a set target temperature Thα for the subsequent recording medium of the large size, at which the feeding, conveying, and a image-forming processing of the recording medium of the large size are restarted.
However, in a design becoming common in recent years, the heat capacity of the heating member is reduced to as low a level as possible and electric power to the heat source (fixing heater) is increased in order to shorten the time (wait time) required to raise the temperature of the heating member functioning as the fixing member from room temperature to a predetermined fixing temperature. As a result, it becomes difficult to suppress the temperature rise in the non-sheet-passing area for the small size during continuous sheet-passing, and the time required to decrease to the uniformly distributed temperature T0 as in the above-mentioned post-rotating tends to become longer. Further, the time required to raise the temporarily reduced temperature of the fixing member up to the subsequent set target temperature Thα is also longer by the time required to decrease the temperature.